Bulk Niobium Research Articles

Structural, magnetic and magnetoelectric properties of Nb substituted Cobalt Ferrite

The structural and magnetic properties of bulk polycrystalline CoFe2O4 and niobium substituted Co1.1Fe1.85Nb0.05O4 prepared by solid state reaction method, are presented in this paper. The lattice parameter of Co1.1Fe1.85Nb0.05O4 is seen to be more than that of CoFe2O4. Saturation magnetization, first order magnetocrystalline anisotropy constant calculated using law of approach to saturation and magnetostriction of Co1.1Fe1.85Nb0.05O4 are seen to be less than those of CoFe2O4. Longitudinal magnetostriction of Co1.1Fe1.85Nb0.05O4 measured at room temperature is −123×10−6 at a field of 2.2kOe, with enhanced strain derivative. Field induced trigonal distortion was observed at 2.2kOe in Co1.1Fe1.85Nb0.05O4 from the analysis of powder XRD patterns. Co1.1Fe1.85Nb0.05O4 exhibits ferroelectric and magnetoelectric properties.

Strong Meissner screening change in superconducting radio frequency cavities due to mild baking

We investigate “hot” regions with anomalous high field dissipation in bulk niobium superconducting radio frequency cavities for particle accelerators by using low energy muon spin rotation (LE-μSR) on corresponding cavity cutouts. We demonstrate that superconducting properties at the hot region are well described by the non-local Pippard/BCS model for niobium in the clean limit with a London penetration depth λL=23±2 nm. In contrast, a cutout sample from the 120 ∘C baked cavity shows a much larger λ>100 nm and a depth dependent mean free path, likely due to gradient in vacancy concentration. We suggest that these vacancies can efficiently trap hydrogen and hence prevent the formation of hydrides responsible for rf losses in hot regions.

Controlled mechanical cleavage of bulk niobium diselenide to nanoscaled sheet, rod, and particle structures for Pt-free dye-sensitized solar cells

A one-step process for the preparation of NbSe2nanostructures under the effects of mechanical forces to be used as Pt replacement CEs in DSSCs is reported.

Oxygen Atom Adsorption on and Diffusion into Nb(110) and Nb(100) from First Principles

To understand the dynamics of oxidation of Nb, we examine the adsorption, absorption, and diffusion of an oxygen atom on, in, and into Nb(110) and Nb(100) surfaces, respectively, using density functional theory. Our calculations predict that the oxygen atom adsorbs on the threefold site on Nb(110) and the fourfold hollow site on Nb(100), and the adsorption energy is −5.08 eV and −5.18 eV, respectively. We find the long and short bridge sites to be transition states for O diffusion on Nb(110), while the on-top site is a rank-2 saddle point. In the subsurface region, the oxygen atom prefers the octahedral site, as in bulk niobium. Our results show also that the O atom is more stable on Nb(110) subsurface than on Nb(100) subsurface. The diffusion of oxygen atoms into niobium surfaces passes through transition states where the oxygen atom is coordinated to four niobium atoms. The diffusion barriers of the oxygen atom into Nb(110) and Nb(100) are 1.81 eV and 2.05 eV, respectively. An analysis of the electronic density of states reveals the emergence of well-localized electronic states below the lowest states of clean Nb surfaces due to d–p orbital hybridization.

Magnetic Shielding Larger Than the Lower Critical Field of Niobium in Multilayers

The current technology in superconducting radio frequency linear accelerators is based on the use of bulk niobium cavities. However, optimization of bulk technology is approaching the accelerating gradient limit set by the thermodynamic critical field of niobium, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> = 200 mT . In order to surpass niobium's ultimately achievable accelerating gradient, it has been proposed to use multilayer coatings to shield bulk niobium from higher fields. These multilayer coatings involve alternating superconducting and insulating layers. The superconductor used in this multilayer structure must have a higher <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> than that of niobium. NbN is one such superconductor that has potential application in these multilayer coatings. Recently, it has been shown that NbN can sufficiently shield an underlying niobium layer. However, this reported shielding has never been shown to be above the lower critical field of niobium. In this work, we present NbN multilayers that for the first time are shown to be capable of shielding an underlying niobium layer beyond the lower critical field of bulk niobium.

Thickness Dependence and Enhancement of $H_{{\rm C}1}$ in Epitaxial $\hbox{MgB}_{2}$ Thin Films

Thin film MgB2 is a promising material for technical improvements in superconducting radio frequency (SRF) technology and applications. At present, bulk niobium SRF accelerating cavities suffer from a fundamental upper limit in maximally sustained accelerating gradients; however, a scheme involving multilayered superstructures consisting of superconducting-insulating-superconducting (SIS) layers has been proposed to overcome this fundamental material limit of 50 MV/m. The SIS multilayer paradigm is reliant upon implementing a thin shielding material with a suitably high HC1, which may prevent early field penetration in a bulk material layer and consequently delay the high field breakdown. It has been predicted that for thin superconducting films - thickness less than the London penetration depth ( ~ 120 nm in the case of MgB2) - the lower critical field HC1 can be enhanced with decreasing thickness. Thus, MgB2, with a high TC and relatively low HC1 value, as compared with Nb, is a prime candidate for such SIS structures. Here we present our study on the microstructure, surface morphology, and superconducting properties on a thickness series of MgB2 thin films and correlate the effects of film thickness and surface morphology on HC1.

Study of nanometric superconducting multilayers for RF field screening applications

Vortex penetration severely limits high field performances in bulk niobium RF cavities for accelerators. To reach higher fields, Gurevich [Appl. Phys. Lett. 88, 012511 (2006)] proposed to deposit nanometric layers (d &amp;lt; λ) to partially screen of the field sensed by niobium. Model (NbN/MgO)n (n = 0 to 4) samples have been deposited on thick Nb layers. This paper presents the first complete characterization set (HC1 and RF surface resistance) of a new family of composite nanostructured superconducting layers deposited on Nb, which are liable to bring a breakthrough in the technology of superconducting accelerating RF cavities.

Roughness analysis applied to niobium thin films grown on MgO(001) surfaces for superconducting radio frequency cavity applications

This paper describes surface studies to address roughness issues inherent to thin film coatings deposited onto superconducting radio frequency (SRF) cavities. This is particularly relevant for multilayered thin film coatings that are being considered as a possible scheme to overcome technical issues and to surpass the fundamental limit of $\ensuremath{\sim}50\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ accelerating gradient achievable with bulk niobium. In 2006, a model by Gurevich [Appl. Phys. Lett. 88, 012511 (2006)] was proposed to overcome this limit that involves coating superconducting layers separated by insulating ones onto the inner walls of the cavities. Thus, we have undertaken a systematic effort to understand the dynamic evolution of the Nb surface under specific deposition thin film conditions onto an insulating surface in order to explore the feasibility of the proposed model. We examine and compare the morphology from two distinct $\mathrm{Nb}/\mathrm{MgO}$ series, each with its own epitaxial registry, at very low growth rates and closely examine the dynamical scaling of the surface features during growth. Further, we apply analysis techniques such as power spectral density to the specific problem of thin film growth and roughness evolution to qualify the set of deposition conditions that lead to successful SRF coatings.

Proximity breakdown of hydrides in superconducting niobium cavities

Many modern and proposed future particle accelerators rely on superconducting radio frequency cavities made of bulk niobium as primary particle accelerating structures. Such cavities suffer from the anomalous field dependence of their quality factors Q0. High field degradation—so-called ‘high field Q-slope’—is so far unexplained even though an empirical cure is known. Here, we propose a mechanism based on the presence of proximity-coupled niobium hydrides that can explain this effect. Furthermore, the same mechanism can be present in any surface-sensitive experiments or superconducting devices involving niobium.

Direct observation of hydrides formation in cavity-grade niobium

Niobium is an important technological superconductor used to make radio frequency cavities for particle accelerators. Using laser confocal microscopy we have directly investigated hydride precipitates formation in cavity-grade niobium at 77 and 140 K. We have found that large hydrides were usually formed after chemical or mechanical treatments, which are known to lead to a strong degradation of the quality factor known as $Q$ disease. From our experiments we can conclude that hydrides causing $Q$ disease are islands with a characteristic thickness of $\ensuremath{\gtrsim}100\text{ }\text{ }\mathrm{nm}$ and in-plane dimensions $1--10\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. Our results show that mechanical polishing uploads a lot of hydrogen into bulk niobium while electropolishing leads to a mild contamination. Vacuum treatments at $600--800\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ are demonstrated to preclude large hydride formation in line with the absence of $Q$ disease in similarly treated cavities.

Diffusion behaviors of hydrogen isotopes in niobium from first-principles

We investigate diffusion behaviors of hydrogen (H), deuterium (D), and tritium (T) in bulk niobium (Nb) using first-principles method. The diffusion energy barrier with quantum-correction has been calculated to be 0.12 eV, which is in good agreement with the experimental value of ∼0.106 eV. According to diffusion theory presented by Wert and Zener, the diffusion pre-exponential factors of H, D, and T are shown to be 7.74×10−8 m2 s−1, 5.47×10−8 m2 s−1 and 4.48×10−8 m2 s−1, respectively. These results are also in quantitative agreement with the corresponding experimental values.

NbN thin films for superconducting radio frequency cavities

NbN thin films have the potential to be incorporated into radio frequency cavities in a multilayer coating to overcome the fundamental field gradient limit of 50 MV m−1 for the bulk niobium based technology that is currently implemented in particle accelerators. In addition to having a larger critical field value than bulk niobium, NbN films develop smoother surfaces which are optimal for cavity performance and lead to fewer losses. Here, we present a study on the correlation of film deposition parameters, surface morphology, microstructure, transport properties and superconducting properties of NbN thin films. We have achieved films with bulk-like lattice parameters and superconducting transition temperatures. These NbN films have a lower surface roughness than similarly grown niobium films of comparable thickness. The potential application of NbN thin films in accelerator cavities is discussed.

Trapped magnetic flux in superconducting niobium samples

Trapped magnetic flux is known to be one cause of residual losses in bulk niobium superconducting radio frequency cavities. In the Meissner state an ambient magnetic field should be expelled from the material. Disturbances such as lattice defects or impurities have the ability to inhibit the expulsion of an external field during the superconducting transition so that the field is trapped. We have investigated the effect the treatment history of bulk niobium has on the trapped flux and which treatment leads to minimal flux trapping. For that purpose, we measured the fraction of trapped magnetic flux in niobium samples representing cavities with different typical treatment histories. The differences between single crystal and polycrystalline material as well as the influence of spatial temperature gradients and different cooling rates were investigated. In addition, the progression of the release of a trapped field during warm-up was studied. We found that heat treatment reduces trapped flux considerably and that single crystal samples trap less flux than polycrystalline niobium. As a consequence, the single crystal sample with $1200\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ baking trapped the smallest amount of field which is about 42%. Moreover, the release of the trapped field during warm-up was observed to progress over a broad temperature range for the baked single crystal samples.

In Situ NbC Particulate-Reinforced Iron Matrix Composite: Microstructure and Abrasive Wear Characteristics

An niobium carbide (NbC) particle-reinforced iron matrix composite was fabricated by compounding gray cast iron with niobium wires through an in situ technique comprised of an infiltration casting process and a subsequent heat treatment. The NbC particles in the reinforcement phase were synthesized in situ through the reaction between niobium from niobium wires and carbon from the graphite phases produced by the heat treatment in gray cast iron. The microstructure and wear-resistance of the bulk niobium particle-reinforced iron matrix composite was studied at different NbC particle volume fractions (8, 15, 22, and 28 vol%), by scanning electron microscope, X-ray diffraction, and a wear tester. The NbC particles were observed to form tiny cuboids and nearly spherical particles which were evenly distributed in the matrix. The particle size of the NbC reinforcement was about 0.3–3.5 μm. The relative wear resistance of the bulk composite increased with increasing NbC volume fraction, and the wear resistance of the composite was 5.9-fold higher than that of the gray cast iron under a 20N load and at a 28 vol% volume fraction. Wear performance of the composite at different NbC particle volume fraction values was also analyzed.

Superconducting Radio-Frequency Technology R&amp;D for Future Accelerator Applications

Superconducting rf (SRF) technology is evolving rapidly, as are its applications. While there is active exploitation of what one may call the current state-of-the-practice, there is also rapid progress in expanding in several dimensions the accessible and useful parameter space. While state-of-the-art performance sometimes outpaces thorough understanding, the improving scientific understanding from active SRF research is clarifying routes to obtain optimum performance from present materials and opening avenues beyond the standard bulk niobium. The improving technical basis understanding is enabling process engineering to improve both performance confidence and reliability and also unit implementation costs. Increasing confidence in the technology enables the engineering of new creative application designs. We attempt to survey this landscape to highlight the potential for future accelerator applications.

Characterization of Field Penetration in Superconducting Multilayers Samples

The best rf accelerating cavities are made of bulk niobium. This technology is now close to it full development, with any further improvement limited by the superconducting properties of Nb itself. In the best performing cavities the rf equatorial magnetic field H is close to the Nb thermodynamic critical field (HC ≈ 200 mT). In 2006 Gurevich proposed the use of nanoscale layers of superconducting materials having HC higher than Nb. This multilayer structure is expected to shield bulk niobium and therefore to increase the cavity breakdown field. In order to explore this pioneering idea, we have studied the penetration of the magnetic field by mean of nanometric sized multi layer structures deposited on flat single crystal substrates. We have deposited high quality model samples by dc magnetron reactive sputtering on R-plane cut sapphire substrates. A 250 nm layer of niobium represents the bulk material as in rf cavities. Such Nb layers were coated with a single or multiple stacks of NbN layers (25 nm or 12 nm) separated by 15 nm MgO barriers, and characterized by X-ray reflectivity and dc transport measurements, dc magnetization curves have been measured by conventional super conducting quantum interference device (SQUID) to determine the first penetration field BC1. For comparison, BC1 was also mea sured with a local probe method based on 3rd harmonic analysis. The Nb samples coated with NbN multi-layers clearly exhibit a higher first penetration field.

Characterization of the Low Temperature Superconductor Niobium Carbonitride

Niobium carbonitride's superior radiation tolerance , coupled with the recently reported increase in its upper critical magnetic field when made nanocrystalline, increases its potential importance in future technological high-field superconductor applications. The maximum transition temperature for the composition NbC <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</sub> is ~17.8 K and its upper critical magnetic field is ~11 T; this increases to ~12 T for the composition NbC <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.2</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.8</sub> ,. Using solid-state processing, we have fabricated microcrystalline bulk niobium carbonitride with a transition temperature of ~ 7.6 K. A comprehensive characterization of this material, which includes susceptibility, resistivity, magnetization, heat capacity and XRD measurements is provided. Comparisons between the heat-treated material and the same material subjected to hot isostatic pressing are made so that the values of the intrinsic fundamental properties can be identified and their sensitivities to different fabrication processes determined.

Note: Radio frequency surface impedance characterization system for superconducting samples at 7.5 GHz

A radio frequency (RF) surface impedance characterization (SIC) system that uses a novel sapphire-loaded niobium cavity operating at 7.5 GHz has been developed as a tool to measure the RF surface impedance of flat superconducting material samples. The SIC system can presently make direct calorimetric RF surface impedance measurements on the central 0.8 cm(2) area of 5 cm diameter disk samples from 2 to 20 K exposed to RF magnetic fields up to 14 mT. To illustrate system utility, we present first measurement results for a bulk niobium sample.

Fermi level spin polarization of polycrystalline thulium by point contact Andreev reflection spectroscopy

The spin polarization near the Fermi level in bulk polycrystalline ferrimagnetic thulium is investigated by means of point contact Andreev reflection (PCAR), in the temperature interval (1.9–9.2 K). The highest polarization measured is P = 0.41(8), with barrier parameter Z = 0.39(8), proximity region gap of Δ1 = 1.26(5) meV and an elevation of the electronic temperature of ΔTe = 3.3(8) K, for a lattice temperature of T = 2.20(5) K, and a contact conductance of G = 330(5) G0. Both polarization and barrier power are found to be constant within the experimental uncertainties in the entire accessible temperature interval and linearly independent, while the proximity gap was found to obey the standard BCS temperature dependence with a TcGap = 10.2(2) K, higher than the TcGap = 9.2 K of bulk niobium. Despite its low ordering temperature, thulium could be suitable candidate for low critical current spin-transfer-torque demonstrations.

Production and separation of 97Ru from 7Li activated natural niobium

Abstract The proton rich 97Ru, a potential candidate for nuclear medicine, was produced for the first time through the natNb(7Li,3n)97Ru reaction. A natural Nb foil of 20ߙmg/cm2 thickness was irradiated with 32ߙMeV 7Li beams to attain the yield of ∼1ߙMBq/μAߙh for 97Ru. The no-carrier-added 97Ru was separated from the bulk niobium by liquid–liquid extraction (LLX) and solid–liquid extraction (SLX) techniques. Liquid cation exchanger, di-(2-ethylhexyl)phosphoric acid (HDEHP) dissolved in cyclohexane was used in LLX and solid cation exchanger, DOWEX-50 was used for SLX in HCl. The LLX technique was found to be superior to the SLX technique with respect to radiochemical yield and purity of the radionuclide 97Ru.